24v to 13.8v Converter for Headlights

[richard5933]

I could use the assistance of an electrical engineer here...Hopefully with all the ham operators in the group someone can lend a bit of advice.

First, the background to the question...

I'm in the slow process of updating the headlights on my coach. The main coach chassis is 24v. The headlights are standard 12v automotive sealed beams. They are wired in four separate circuits for redundancy so that if something fails, only one something fails. Eventually I'll update to LED headlights, but first I'm just focused on getting what I've got working properly. (The headlight buckets won't easily accept LED headlights due to the extra depth required).

The way that GM converted the 24v to 12v was through the use of a bank of resistors. At the moment, after going through every connection with a fine tooth comb, the highest output I can get from the resistors is 11.8v which explains why my headlight output is dismal. I assume the resistors have failed due to age. The input voltage is correct and ranges from over 24v with engine off to about 28v with alternator running at full engine speed. Photo attached is the OEM resistor panel that I'm replacing.

Since the sealed beam headlights work best at about 13.8v, I decided to upgrade from the resistors to a modern 24v-to-13.8v converter. One for each of the four circuits. This is the unit I'm going with: https://www.amazon.com/gp/product/B01LY8D7U0/ref=ppx_yo_dt_b_asin_title_o01__o00_s00?ie=UTF8&psc=1

At 40 amps each, these have more capacity than I need and should leave plenty of headroom in the circuit. They will provide a steady 13.8v to the headlights regardless of engine speed.

Everything seemed good. Until I saw this line in the instructions:

It is recommended to use stable DC power for the input power supply If you use the pulsed DC power provided by the generator, please filter it with capacitor before connecting it to the converter.

I'm not sure exactly what they're talking about here. Is the DC power in the coach 'stable'? What is pulsed DC power? Would they be talking about the output from the coach engine's alternator?

Is this something I need to be concerned about? I am thinking that they are trying to avoid having the converter output pulsate, causing things like LED lights to pulsate. I'm going to use incandescent bulbs for now, which don't usually exhibit problems quite as much as LEDs. But, I really don't want to have my headlights pulsating.

Seems to me that the output from the bus alternator will be tempered quite a bit by the existence of the two 8D batteries in the circuit, so that by the time these converters get the input it will be pretty smooth.

I'm sure that if I need the capacitors, more is involved than just slapping a capacitor across the input lines. Maybe something like this: https://www.amazon.com/Suppressor-Single-Phase-Line-Conditioner-JREle-CW4L2-20A-S/dp/B073MCGBP5/ref=pd_lpo_vtph_23_bs_t_1?_encoding=UTF8&psc=1&refRID=TWEJRPJQRNY0WP0427DN

I'm at about at the end of my knowledge base once we get into the nitty gritty of modifying electronic circuits. This is the portion of my ham operator ticket I barely scraped by with, so I could use some help.

[jleamont]

15 minutes ago, richard5933 said:

I'm not sure exactly what they're talking about here. Is the DC power in the coach 'stable'? What is pulsed DC power? Would they be talking about the output from the coach engine's alternator?

Richard, since you are pulling from the batteries you have adequate filtration there. If you were tapped directly off of the alternator or old school generator than those instructions would apply. 

LED headlights would be a fantastic improvement, post some photos of the headlight buckets.

[bm02tj]

Why not put a 12v alternator and a series parallel switch for the 24 start same as transport trucks   

[dickandlois]

Richard, Your 24 volt system should be a very stable source of power ! So there should be no need for a cap to filter the things.

The resister divider you picture is old school but very reliable - only time one of them fail is when something shorts out. The lights should be fused to prevent any damage though. Oxidation or loose connections are the most common problem in the circuit pictured. 

The box you are looking at is a DC to DC converter, that is designed to convert 24 volts to 12 volts. A very Common practice in today's world. 

Are you saying that one of the headlights is not working or is just dimmer then the other?

FYI. I got the last list of items you are hoping to get some info on and will keep looking around, sending things as I find them. Will keep you in the loop.  

Rich.

[richard5933]

2 hours ago, bm02tj said:

Why not put a 12v alternator and a series parallel switch for the 24 start same as transport trucks   

Not so easy in our case. No free pulley, and no room where we'd need it. Besides, other than the headlights nothing on the coach chassis system runs on 12v. I suspect that this change was made so that the easier-to-obtain 12v sealed beams could be used. The 24v headlights are really hard to find.

 

7 hours ago, jleamont said:

Richard, since you are pulling from the batteries you have adequate filtration there. If you were tapped directly off of the alternator or old school generator than those instructions would apply. 

LED headlights would be a fantastic improvement, post some photos of the headlight buckets.

LED are definitely in the plans for the future, but not this year. The budget and my desire to take that on just aren't there. No photos of the headlight buckets on hand, and won't be able to get to them till spring. The problem I'm seeing with putting LED headlights into our bus is going to be the depth of the LED headlights, especially with the cooling fins on the back side of them. I'd have to do some cutting to the buckets, and even then I've seen videos of others having problems getting them to fit. We've got quad 5-3/4" headlights, which are hard to find in LED to begin with, especially for applications like ours.

That said, if you know of DOT-approved 5-3/4" headlights which are shallow enough to work I'm all ears.

2 hours ago, DickandLois said:

The resister divider you picture is old school but very reliable - only time one of them fail is when something shorts out. The lights should be fused to prevent any damage though. Oxidation or loose connections are the most common problem in the circuit pictured. 

The box you are looking at is a DC to DC converter, that is designed to convert 24 volts to 12 volts. A very Common practice in today's world. 

Are you saying that one of the headlights is not working or is just dimmer then the other?

The headlights had a very dim light - all four of them equally. First step was to get new sealed beams, as the originals must have been in there for years. The mirrors were yellowed and not doing their job. New sealed beams helped greatly, but things were still too dim.

Voltage going to the resistor panel is where it should be. All four headlight circuits are providing over 24v with engine off and up to 28 with it running. The problem was apparently downstream from that. I removed the resistor panel and cleaned every connector and terminal. Everything visibly looked good - no broken resistors or wires, no corrosion, etc. Best I could get for output was 11.9v which is simply not enough for the sealed beams to shine bright enough.

My guess is that in the nearly 50 years these things have been in place there has been some internal breakdown which has upped the resistance. I thought about getting new resistors to match the old ones, but there was really no sense in doing that when I could just replace the whole thing with the DC-to-DC converters and have steady 13.8v regardless of engine speed.

All that's left now is to wait for a warm enough day to work outside (40+ degrees for me) and get to it. From the comments above it seems that I should be good to go just replacing the resistors with the new converters. I plan mount them on the same metal plate and tie into the harness using a new 8-wire connector (actually, will probably go with two Packard type 56 four-wire connectors).

Thanks for the input.

[f433921]

It's difficult to determine if the resistors in the resistor bank are wired in series or parallel. Bases on the information that you have provides the voltage drop across the resistor bank appears to between 12-16 volts depending on the alternator output voltage.  I assume there is not any voltage regulation on the output of the resistor bank other than difference of load from LOW beam to HI beam and the output voltage of the alternator.

If the voltage drop is only a function of OHM LAW  E=IR you could change the resistance of one or more resistor in the resistor bank to reduce the voltage across resistor bank.

Jim

 

 

[wolfe10]

Though using resistors to lower voltage may be simple, I sure hate to see an alternator work harder to have that energy turned into HEAT in the resistors.

[desertdeals69]

LED bulbs run on 12-24 volts.

[kaypsmith]

Richard, here is what MCI uses,  which is what is used on my bus. https://www.ebay.com/itm/Vanner-High-Efficiency-80-Amp-Battery-Equalizer-65-80-/292359153516?hash=item4411f6a76c

All you need to do is install this unit in your coach, hardwire the headlight circuit to the first battery for 12 volts. The equalizer in turn monitors both batteries to keep the two charging and discharging at equal rates to make sure that both batteries are working correctly. Wiring is very simple I can copy and send you the schematic from my coach manual. The headlights and ecm are the only two devices that use 12 volt on my 24 volt system, but I also charge my house 12 volt house system with this device while traveling.

[kaypsmith]

3 minutes ago, desertdeals69 said:

LED bulbs run on 12-24 volts.

Correct only if you verify 10/30 volt operation. They are available in both 12 volt and 10/30.

[dickandlois]

Richard, 

Voltage going to the resistor panel is where it should be. All four headlight circuits are providing over 24v with engine off and up to 28 with it running. The problem was apparently downstream from that. I removed the resistor panel and cleaned every connector and terminal. Everything visibly looked good - no broken resistors or wires, no corrosion, etc. Best I could get for output was 11.9v which is simply not enough for the sealed beams to shine bright enough.

Lets try a different approach. With any one of the headlights removed, what is the dc voltage reading to a good chassis ground? What is the reading between the head light wiring  ground and the 12 volt light feed. this is just a voltage reading. subtract one voltage from the other, what is the difference? 

Got a good digital meter?

What is the  resistance reading between the headlight socket pins.?  not plugged in, No Power / light plugged in,  lights power  turned off,    Measure the filament resistance again . Write down the number(s).    

Trying to see if there is a voltage drop difference between the harness wiring and a know good ground at a point on the frame. with some known numbers -seeing what you have /com paired to what the math come out to. Where is the voltage drop / resistance being injected/added?

Rich.

[dickandlois]

31 minutes ago, wolfe10 said:

Though using resistors to lower voltage may be simple, I sure hate to see an alternator work harder to have that energy turned into HEAT in the resistors.

Historical equipment was built to work - no real consideration of efficiency. That is why AC won over DC, but big generators did not travel  well. 

Rich.  

[richard5933]

Rich - I did run through that routine during the summer when I first realized I had a problem. The voltage leaving the resistor panel and the voltage at the headlamp sockets were the same. If I remember correctly, about 11.9v.

The factory resistor panel is actually double layered. Hard to see in the photo, but each of those 6 resistors is actually two stacked on top of each other. They are wired in a combination of series/parallel, and in theory the sets are matched to the headlights to provide the necessary resistance to allow the headlamps to burn at maximum brightness.

I tried to find a way to make the OEM resistor panel work properly, but in the end decided that it just didn't make sense when these converters were available. I also hate to see the alternator work so hard only to be providing heat for the tool compartment with all these resistors. Not only will they be more efficient, these converters will provide steady 13.8v to the headlights (I hope).

Sounds like for now I am going to try and wire them without any capacitors. If there is noticeable flicker or pulsation I'll re-address things and add something to the output of the converters to smooth things out.

[richard5933]

34 minutes ago, kaypsmith said:

Richard, here is what MCI uses,  which is what is used on my bus. https://www.ebay.com/itm/Vanner-High-Efficiency-80-Amp-Battery-Equalizer-65-80-/292359153516?hash=item4411f6a76c

All you need to do is install this unit in your coach, hardwire the headlight circuit to the first battery for 12 volts. The equalizer in turn monitors both batteries to keep the two charging and discharging at equal rates to make sure that both batteries are working correctly. Wiring is very simple I can copy and send you the schematic from my coach manual. The headlights and ecm are the only two devices that use 12 volt on my 24 volt system, but I also charge my house 12 volt house system with this device while traveling.

I thought about that - problem would be having to run new wiring from back to front to power the headlights. Also, they've got the headlight circuits done with so much redundancy that I really hate to touch it. The relays for the headlights and the running lights are intertwined somehow, and I have yet to fully grasp what they were doing. I have figured out though that I've got lots of redundancy and it just seemed more simple to replace the antiquated resistor panel with the modern DC-DC converters and call it done.

For what it's worth, we use our 24v system to charge our 12v house batteries with one of these: https://www.sterling-power-usa.com/SterlingPower24volt-to-12volt-dc-to-dc-batterytobatterycharger.aspx

Great part about that is that it's a smart charger and will charge the house batteries as they need independently of what the bus's voltage regulator is doing. It can put up to 70 amps into our house batteries to charge while on the road. So far so good.

[dickandlois]

 I did run through that routine during the summer when I first realized I had a problem. The voltage leaving the resistor panel and the voltage at the headlamp sockets were the same. If I remember correctly, about 11.9v.

That being the case  you should be able to pick up close to 2 volts with a DC to DC converter. That extra would make a big difference in candle power.

Rich.

 

[richard5933]

1 minute ago, DickandLois said:

 I did run through that routine during the summer when I first realized I had a problem. The voltage leaving the resistor panel and the voltage at the headlamp sockets were the same. If I remember correctly, about 11.9v.

That being the case  you should be able to pick up close to 2 volts with a DC to DC converter. That extra would make a big difference in candle power.

Rich.

 

Yeah - that's my hope. Should help us get where we're going. Realized we had a problem when I pulled in the TT campground in Lancaster PA after dark - there was no street lighting to speak of and we could barely see enough to find our site. This was the first trip that we've had more than a couple of minutes after dark on anything other than well-lit roads, which explains why I hadn't noticed it before.

[dickandlois]

48 minutes ago, desertdeals69 said:

LED bulbs run on 12-24 volts.

The real problem appears to be room to mount the needed LEDs . Close quarters and heat of 24 volt LEDs could lead to short life span.

Rich.

[dickandlois]

Richard, This device is more for AC filtering and might cause other issues.

Noise Suppressor Power EMI Filter Termianl Single-Phase Line-Conditioner JREle AC 115/250V 20A CW4L2-20A-S

Rich.

You posted a link for a DC to DC box and now I'm having problems finding the link. 

darned old organic computer circuits!

[kaypsmith]

Or just add a nice set of driving led 10/30 volt to the front end for those places that really need more light, usually easy to conceal in order to not take away from the antiquity of the coach. These type work great on any voltage from 10 to 30 volts, if interested, I have a source.

[richard5933]

2 minutes ago, kaypsmith said:

Or just add a nice set of driving led 10/30 volt to the front end for those places that really need more light, usually easy to conceal in order to not take away from the antiquity of the coach. These type work great on any voltage from 10 to 30 volts, if interested, I have a source.

I've got a set of LED 'driving lights' installed under the bumper. They're not DOT approved and send light all over the place. They do help us find our way in campgrounds or on back roads, but I can't use them on public roads. They run directly from the 24v circuits. I installed them at the end of last season as a stop-gap till I got the headlights working better.

[richard5933]

Finally got the new 24v-to-13.8v converter panel built. Photo shows the new next to the old resistor panel. I'm waiting for one old-school (Pack Con II) connector to arrive on Friday so I can plug it into the harness where the old panel was.

The OEM wiring has a separate circuit breaker and power feed for each of the four headlights. Just gotta love the redundancy they built into these old coaches. I followed the same four-circuit system to maintain the redundancy, and I used converters which have far higher capacity than needed (they're rated at 40a each) to ensure lots of overhead since capacity will be reduced as these things heat up.

I did some preliminary testing and found absolutely no discernible flicker at all. The light output from the headlights was also considerably higher, as it should be raising the voltage feeding them from about 12v to 13.8v.

I'll post some lights-on photos once everything is buttoned up.

[dickandlois]

Richard, That Looks like what I expected to see on the first go around.  

      Please Note!that I have been derailed some for health reasons and did not see you reply to my posting of the original information request.

Starting on a mending curve for now - with some other issues to be addressed later.  The thing is life never seams to loose any of its share of challenges.

Rich.

[wildebill308]

58 minutes ago, DickandLois said:

The thing is life never seams to loose any of its share of challenges.

That is true get better soon.

Bill

[richard5933]

1 hour ago, DickandLois said:

I have been derailed some for health reasons

Sorry to hear about this...here's to a quick and full recovery!

[rayin]

Richard it looks like you intend to run all new wiring to the headlamps, which is a great idea as wiring can deteriorate over time and create higher resistance. Thought you might get a chuckle from this: